1. Field of the Invention
The invention relates to a transparent, biaxially oriented and heat-set film having one or more layers and comprising, as main constituent, at least one crystallizable thermoplastic. It further relates to the use of the film and to a process for its production.
2. Description of the Related Art
Transparent, biaxially oriented films made from crystallizable thermoplastics, in particular from crystallizable polyesters, are known and have been described on many occasions.
For example, U.S. Pat. No. 4,399,179 discloses a polyester film laminate with a biaxially oriented, transparent base layer and with at least one monoaxially or biaxially oriented, matt outer layer, the matt layer essentially consisting of a polyethylene terephthalate copolymer having units derived from oligo- or polyethylene glycol , with inert particles. The particles (preferably of silica, kaolin, talc, zinc oxide, aluminum oxide, calcium carbonate or barium sulfate) mostly have diameters of from 0.3 to 20 μm. The proportion of these in the outer layer is generally from 3 to 40% by weight. Base layer and/or outer layer may moreover comprise antioxidants, UV absorbers, pigments, or dyes. The particle-roughened surface of the outer layer is writable.
The biaxially oriented and heat-set polyester film of GB-A 1 465 973 has more than one layer and encompasses a layer made from transparent polyethylene terephthalate and a layer made from a copolyester, which is likewise transparent. Rollers can be used to give the surface of the copolyester layer a rough structure, making the film writable.
EP-A 035 835 describes a biaxially oriented and heat-set polyester film having more than one layer, encompassing a layer made from a highly crystalline polyester and, bonded thereto, a sealable layer made from a substantively amorphous, linear polyester. The latter layer comprises finely distributed particles, the average diameter of the particles being greater than the thickness of the layer. These particles form surface protrusions which prevent undesirable blocking or adherence to rolls or guiding systems. The result is better winding and processing of the film. The sealing performance of the film is impaired by choosing particles whose diameter is greater than the thickness of the sealable layer, at the concentrations given in the examples. The sealed seam strength of the sealed film at 140° C. is in the range from 63 to 120 N/m (0.97 N/15 mm to 1.8 N/15 mm of film width).
EP-A 432 886 describes a coextruded film with a polyester base layer, and with an outer layer made from a sealable polyester, and with a reverse-side polyacrylate coating. The sealable outer layer may be composed of a copolyester having units derived from isophthalic acid and terephthalic acid. The reverse-side coating gives the film improved processing performance. The sealed seam strength is measured at 140° C. For a sealable layer of 11 μm thickness, the sealed seam strength given is 761.5 N/m (11.4 N/15 mm). A disadvantage of the reverse-side acrylate coating is that this side no longer has sealability to the sealable outer layer. The uses of the film are therefore very restricted.
Another coextruded, sealable polyester film having more than one layer is described in EP-A 515 096. The sealable layer additionally comprises pigmentation particles, preferably silica gel particles. The particles may also be applied to the film after extrusion, for example by coating with an aqueous silica gel dispersion. This method is intended to give a film whose sealing properties have been retained and which processes well. The reverse side comprises only very few particles, most of which pass into this layer via the regrind. The sealed seam strength is measured at 140° C. and is above 200 N/m (3 N/15 mm). The sealed seam strength given for a sealable layer of 3 μm thickness is 275 N/m (4.125 N/15 mm).
WO 98/06575 discloses a coextruded polyester film having more than one layer and encompassing a sealable outer layer and a non-sealable base layer. This base layer may have been built up from one or more layers, the interior layer being in contact with the sealable layer. The other (exterior) layer then forms the second, non-sealable outer layer. Here too, the sealable outer layer may be composed of copolyesters having units derived from isophthalic acid and terephthalic acid. However, no antiblocking particles are present in the outer layer. The film also comprises at least one UV absorber, present in a proportion of from 0.1 to 10% by weight in the base layer. The UV absorbers used in this instance are zinc oxide particles or titanium dioxide particles, in each case with an average diameter below 200 nm, but preferably triazines, e.g. ®Tinuvin 1577 from Ciba. The base layer has conventional antiblocking agents. The film has good sealability, but not the desired processing performance, and also has shortcomings in its optical properties.
Layers made from copolyester can be produced by applying an appropriate aqueous dispersion. For example, EP 144 878 describes a polyester film which, on at least one side, has a continuous coating made from the copolyester. The dispersion is applied to the film prior to orientation or, respectively, prior to the final step of orientation. The polyester coating is composed of a condensation product of various monomers capable of forming polyesters, for example isophthalic acid, aliphatic dicarboxylic acids, sulfomonomers, and aliphatic or cycloaliphatic glycols.
DE-A 23 46 787 discloses, inter alia, flame-retardant films made from linear polyesters, modified with carboxyphosphinic acids. However, production of these films is attended by a variety of problems, for example, the raw material is very susceptible to hydrolysis and requires very thorough predrying. When the raw material is dried using prior art dryers it cakes, and production of a film, is possible only under very difficult conditions. The films produced, under extreme and uneconomic conditions, also embrittle at high temperatures. The associated decline in mechanical properties is so severe as to make the film unusable. This embrittlement arises after as little as 48 hours at high temperature.
Prior art films are generally insufficiently resistant to aqueous media, and they, and the products produced from them, are therefore unsuitable for applications in contact with moisture or water. Here, again, mechanical properties deteriorate to the extent that the films can become completely unusable.